Add TestableHttpClientFactory to support IHttpClientFactory usage

[sandermvanvliet-stack]

This PR adds the TestableHttpClientFactory to help with scenarios where you are using a IHttpClientFactory in your code:

The code under test:

public class TheRealComponent
{
    private readonly IHttpClientFactory _httpClientFactory;

    public TheRealComponent(IHttpClientFactory httpClientFactory)
    {
        _httpClientFactory = httpClientFactory;
    }

    public async Task<string> ExecuteAsync()
    {
        var httpClient = _httpClientFactory.CreateClient("TheNameOfTheClient");
        return await httpClient.GetStringAsync("https://example.com");
    }
}

The test:

var httpClientFactory = new TestableHttpClientFactory();
var handler = httpClientFactory.ConfigureClient("TheNameOfTheClient");

handler
    .RespondTo()
    .Get()
    .ForUrl("https://example.com")
    .With(HttpStatus.OK)
    .AndContent("text/plain", "Hello, world!");

var sut = new TheRealComponent(httpClientFactory);

var result = await sut.ExecuteAsync();

result
    .Should()
    .Be("Hello, world!");

The TestableHttpClientFactory will return a new HttpClient instance which is backed by the TestableMessageHandler you've configured in the test.